Modification of UHMW polyethylene with TLCP can not only improve the fluidity during processing, but also maintain high tensile strength and impact strength, and greatly improve the wear resistance. Polymerization filling process in polymer synthesis is a new polymerization method, which is to treat the filler to form active centers on the surface of its particles. In the polymerization process, olefin monomers such as ethylene and propylene are polymerized on the surface of filler particles to form a resin that tightly wraps the particles, and finally a composite material with unique properties is obtained. In addition to the characteristics of blended composite materials, it also has its own characteristics: firstly, it does not need to melt polyethylene resin, and can keep the shape of filler to prepare powdery or fibrous composite materials; Secondly, the composite material is not limited by the filler/resin composition ratio, and the filler content can generally be set at will; In addition, the obtained composite material is a homogeneous composition, which is not limited by the specific gravity and shape of the filler.
Compared with hot-melt * * * mixed materials, the filler particles in UHMW- polyethylene composites prepared by polymerization filling process have better dispersibility and better interface between particles and polymer matrix. This makes the tensile strength and impact strength of the composite material not much different from UHMW-PE, but far better than that of the * * * mixed material, especially in the case of high filling. The hardness and bending strength of the composite, especially the bending modulus, are much higher than that of pure UHMW-PE, and it is especially suitable for bearing, shaft seat and other stressed parts. In addition, the thermodynamic properties of the composites have been improved: the Vicat softening temperature has increased by nearly 30℃, the thermal deformation temperature has increased by nearly 20℃, and the linear expansion coefficient has decreased by more than 20%. Therefore, this material can be used in high temperature occasions and is suitable for manufacturing mechanical parts with high precision requirements such as bearings, bushings and gears.
Polymer filling technology can also control the molecular weight of ultra-high molecular weight polyethylene (UHMW-PE) by introducing hydrogen or other chain transfer agents into the polymerization system, making the resin easy to process.
U.S. patents use fillers with acid-neutral surfaces, such as hydrated alumina, silica, water-insoluble silicate, calcium carbonate, basic sodium aluminum carbonate, hydroxyl wollastonite and calcium phosphate, to manufacture UHMW-PE composites filled by homogeneous polymerization with high modulus. Another patent pointed out that under the conditions of 60℃, 65438±0.3 MPa and catalyst, ultra-high molecular weight polyethylene (UHMW-PE) was polymerized on the surface of alumina dried in heptane, and a uniform composite with high modulus could be obtained. Ultra-high molecular weight polyethylene (UHMW-PE) composites were synthesized by Qilu Petrochemical Company Research Institute with diatomite and kaolin as fillers respectively. Ultra-high molecular weight polyethylene (UHMW-PE) can not only be blended with plastics to improve its processability, but also obtain other properties. Among them, PP/ UHMW-PE alloy is the most prominent.
Usually, the toughening of polymers is to introduce flexible segments into resin to form composite materials (such as rubber-plastic mixture), and its toughening mechanism is "multiple silver plating mechanism". In PP/ UHMW-PE system, UHMW-PE has obvious toughening effect on PP, which cannot be explained by "multiple cracks" theory. 1993 reported for the first time that ultra-high molecular weight polyethylene (UHMW-PE) successfully toughened polypropylene in China. When the content of UHMW- PE is 15%, the notched impact strength of * * mixture is more than 2 times higher than that of pure PP. It is also reported that the impact strength of UHMW-PE mixed with * * * polypropylene containing ethylene segments is more than 1 time higher when the content of UHMW-PE is 25%. The explanation for the above phenomenon is "network toughening mechanism".
The submicroscopic phase of PP/ UHMW-PE * * * mixed system is bicontinuous, UHMW-PE molecules and long-chain PP molecules * * * form a * * mixed network, and the rest PP forms a PP network, which is interwoven into a "linear interpenetrating network". The * * * hybrid network plays a skeleton role in the material and provides mechanical strength for the material. When impacted by external force, it will undergo great deformation to absorb external energy and play a toughening role. The more complete and dense the network, the better the toughening effect.
In order to ensure the formation of "linear interpenetrating network" structure, ultra-high molecular weight polyethylene (UHMW-PE) must be dispersed in PP matrix at excimer level, which puts forward higher requirements for * * * mixing mode. Beijing university of chemical technology's research found that UHMW-PE can be uniformly dispersed in PP matrix by four-screw extruder, but the mixing effect of twin-screw extruder is not good.
EPDM can compatibilize PP/ UHMW-PE alloy. Because EPDM has the same two main links as PP and UHMW-PE, it has good affinity with both materials and is easy to disperse on the two-phase interface when mixed. EPDM can insert, divide and refine the composite crystal, which is beneficial to improve the toughness of the material and greatly improve the notched impact strength.
In addition, UHMW-PE can also be alloyed with rubber to obtain better mechanical properties than pure rubber, such as friction resistance, tensile strength and elongation at break. Among them, during the mixing process, the rubber is vulcanized at a temperature higher than the softening point of UHMW- polyethylene. Ultra-high molecular weight polyethylene (UHMW-PE) can be vulcanized and compounded with various rubbers (or rubber-plastic alloys) to make modified PE sheets, which can be further compounded with metal sheets to make composite materials. In addition, ultra-high molecular weight polyethylene (UHMW-PE) can be compounded on the plastic surface to improve the impact resistance.